This invention relates to a long-lasting phosphor, a powdered long-lasting phosphor and a method for manufacturing the powdered long-lasting phosphor.
Fluorescence is a phenomenon that a material emits visible ray when excited by an excitation source applied from outside. A fluorescent lamp, a discharge tube and a cathode ray tube utilize fluorescence. A material which emits fluorescence is called a phosphor. When light emitted by a phosphor lasts after stoppage of excitation for duration of time sufficient for the light to be perceived by the eye, i.e., about 0.1 second or longer, the light is called phosphorescence. A phosphor which has a long persistent phosphorescence lasting for several hours at room temperature is called a long-lasting phosphor or a light storage phosphor.
As such long-lasting phosphors, known in the art are sulfides and oxides. Main long-lasting phosphor products used in the past have been those having a sulfide such as ZnS:Cu as their base crystal. They however are disadvantageous in that their after-glow lasts for a relatively short duration of time, i.e., about three hours at the longest. Since a decomposition reaction of ZnS+H.sub.2 O.fwdarw.Zn+H.sub.2 S takes place under the coexistence of ultraviolet ray contained in the sunlight and moisture contained in the air, the phosphor becomes black and, as a result, the after-glow characteristics significantly deteriorate within a relatively short period of time. For this reason, this type of phosphor has only limited applications such as a luminous watch and a night-time display of a location in a house.
In addition to these long-lasting phosphors, there have recently been developed alkaline earth metal aluminate long-lasting phosphors having alkaline earth metal aluminate as their base crystal and comprising divalent europium as an activator and one or more rare earth elements as a co-activator (Japanese Patent No. 2543825 and Japanese Patent No. 2697688). By normally adding phosphoric acid or boric acid as a flux, these long-lasting phosphors exhibit phosphorescence brightness and lifetime which are about ten times as large as the sulfide type long-lasting phosphors. These long-lasting phosphors, however, become a hard mass by the use of such flux with resulting difficulty in crushing the long-lasting phosphors. In practice, a long-lasting phosphor is often crushed to powder and used as a powdered material mixed with resin, glass, ink or ceramics material. Therefore, in the course of crushing by a powerful crusher, crushing media are mixed as impurities which become obstacles at the light absorbing and light emitting time. Thus, these long-lasting phosphors are disadvantageous in the difficulty in crushing.
Further, when these long lasting phosphors are dispersed in an aqueous solution, pH of the aqueous solution becomes strongly alkaline and, as a result, dispersion of particles of the long-lasting phosphors becomes insufficient which changes physical properties such as viscosity of the aqueous solution. This poses a serious problem, for example, in the manufacture of inks.
These alkaline earth metal aluminate long-lasting phosphors are generally manufactured by sintering of mixed powdered materials at a high temperature. More specifically, highly pure materials such as oxides, carbonates and nitrates are crushed and mixed with a ball mill and then sintered in a reducing atmosphere such as hydrogen gas. Alkaline earth metal aluminate long-lasting phosphor obtained by the sintering is then crushed to particles of a predetermined particle diameter. In some case, the crushed long-lasting phosphor is rinsed and dried for removing flux which has been added for improving the sintering efficiency. Lastly, the crushed particles are classified to provide a product required.
The alkaline earth metal aluminate long-lasting phosphor thus produced has a wide distribution of particle diameter, since it is produced through the crushing process. This results in irregularity in the shape of particles of the product and, besides, contains a significant amount of fine powdery particles which have been produced during the crushing process.
For overcoming this disadvantage of the alkaline earth metal aluminate long-lasting phosphors, an alkaline earth metal silicate (Sr--Mg--Si--O:Eu.Ln) long-lasting phosphor has been developed. Japanese Patent Application Laid-open Publication No. Hei 9-194833 discloses that an alkaline earth metal silicate long-lasting phosphor added with a small amount of Dy as a co-activator exhibits high phosphorescence brightness and long life and, if halogen is added further, exhibits higher phosphorescence brightness and longer life. This long-lasting phosphor is excellent in phosphorescence brightness, life, chemical durability and resistance to light. When, however, this long-lasting phosphor is manufactured by using halogen, halogen gas which is known as a poisonous gas is generated and this is undesirable.
PCT/CN97/00143 Publication discloses an alkaline earth metal silicate long-lasting phosphor which is added with boric acid or phosphoric acid as a flux. Generation of a poisonous gas can be avoided by this method but the sintered product produced by this method becomes a mass which is as hard as the above described alkaline earth metal aluminate long-lasting phosphors and therefore leave the problem discussed above.
Phosphorescence characteristics, particularly phosphorescence brightness, of these recently developed alkaline earth metal aluminate or silicate long-lasting phosphors are influenced by their composition and particle diameter. As to the particle diameter, there is the tendency that, if the particle diameter becomes smaller than 100 .mu.m, phosphorescence brightness decreases proportionally. The mechanism of this phenomenon is not known, though it is conceivable that the crystalline state of the particle surface of long-lasting phosphor is deteriorated due to collapse or distortion of the crystal lattice caused by crushing and, as a result, the state of electrons about europium is deviated from a proper state resulting in deterioration in the functions of excitation and light emission or, alternatively, it is not the surface layer of each particle but the bulk itself that contributes to emission of light and, as the particle diameter decreases, the ratio of the surface layer relatively increases which results in deterioration in the phosphorescence brightness. In any case, it is an established fact that decrease in the particle diameter results in decrease in phosphorescence brightness and, accordingly, if it is desired to obtain an alkaline earth metal aluminate or silicate long-lasting phosphor having high phosphorescence brightness, consideration must be given not only to the composition but also to the particle diameter and distribution of particle diameter.
Further, when characteristics required for a pigment are considered, the shape of the particle in addition to the particle diameter and distribution of particle diameter must be taken into consideration. Particularly, mixing of fine powdery particles and increase in the relative surface area due to an irregular particle shape cause insufficient dispersion of the pigment and increase in the amount of oil absorbed whereas mixing of coarse particles cause insufficient dispersion and reduction in coloring and concealing characteristics of the pigment.
Additionally, these alkaline earth metal aluminate or silicate long-lasting phosphors have the following problems:
(1) Since materials before sintering are mixture of powdered materials in the form of mixture of solids, it is difficult to mix the respective ingredients uniformly.
(2) In the course of mixing materials, impurities from mixing media are mixed which adversely affects emission characteristics of the long-lasting phosphors. Particularly, in the course of crushing process after sintering also, the emission characteristics are deteriorated due to mixing of impurities from the crushing media.
(3) The long-lasting phosphors obtained by crushing have a wide particle diameter distribution and therefore classification is necessary for obtaining particles of a desired average particle diameter. This complicates the manufacturing process. Moreover, fine powdery particles are deposited onto the surface of coarse particles and such fine powdery particles cannot be removed by classification, it becomes difficult to obtain a desired degree of particle. Furthermore, since fine powdery particles which have low light emission intensity cover the surface of coarse particles, the light emission intensity as a whole is also reduced.
It is, therefore, an object of the invention to provide a long-lasting phosphor which has a narrow particle diameter distribution, contains as little fine powdery particles as possible, exhibits no decrease in phosphorescence brightness due to mixing and crushing, has a particle shape which is suitable as a pigment and is uniform at the molecular level without requiring a complicated process as in the above described general manufacturing method.
It is another object of the invention to provide a novel alkaline earth metal aluminate or alkaline earth metal silicate long-lasting phosphor in which no poisonous gas is produced during manufacturing of a sintered long-lasting phosphor, and the sintered mass can be easily crushed and which is excited efficiently by the sunlight or fluorescent lamp of day-light color, and has high phosphorescence brightness and long life.